Blood is the fluid of life carrying oxygen and nutrient, and when necessary, drugs/pharmaceuticals throughout the body. Following injury and/or during surgery, blood levels or blood components may need to be increased in an individual to sustain life. Blood and blood components taken from donors. However, in the blood of an infected donor, pathogenic (disease-causing) microbes may be present. Screening processes can remove tainted blood from the blood and blood-component supply, but some donors may have been infected only recently and their blood pathogens may not yet be at a high enough concentration to be detected by the screening process. These donors increase the risk of transfusion-transmission of injection.
The risk from pathogen-contaminated blood can be reduced via various sterilization techniques. Photodynamic therapy (PDT) using light, endogenous oxygen, and a photosensitizer has been successfully utilized in the treatment of cancer and the same principles can be applied to the removal of microbes from blood. Irradiation with wavelengths of light absorbed by the photosensitizer produces singlet oxygen through the interaction of the triplet excited state of photosensitizer with ground-state oxygen, which is also a triplet. In the treatment of blood and blood products, photodynamic therapy is known as Photodynamic Antimicrobial Chemotherapy (PACT). However, compounds which are used for the treatment of blood products can damage to the blood cells by causing hemolysis, especially after irradiation.
Various dyes have been used with mixed success in PDT and PACT. A structure at the heart of many dyes, and indeed, many important chromophores in chemistry and biology, is the xanthylium nucleus. The rhodamine and rosamine dyes (shown in FIG. 1) are representative of the xanthylium class and have been used as laser dyes, fluorescent labels, and fluorescence emission standards where their high fluorescence quantum yields and photostabilities are exploited. [1-5] The rhodamine dyes are also useful fluorescence probes in cell biology studies, showing specific fluorescence staining of mitochondria and other cell organelles. [6,7] The rhodamines have been found to accumulate selectively in carcinoma cells [8-10] and to be toxic to cancer cells both in vitro [10] and in vivo. [11]
One area where the xanthylium dyes have been minimally utilized is photodynamic therapy (PDT), where their tumor specificity might truly be exploited. [12,13] PDT is a treatment for various cancers that utilizes the combination of a tumor-specific photosensitizer, light, and molecular oxygen to induce cellular toxicity, presumably via the generation of singlet oxygen. [12] While rhodamine and rosamine dyes exhibit selective uptake in cancer cells, they are poor producers of excited-state triplets [14] and, consequently, of singlet oxygen. The poor triplet production limits the use of rhodamine and rosamine dyes as photosensitizers PDT. Furthermore, the rhodamines and rosamines absorb light of wavelengths too short for effective penetration in tissue. [12]
Heavy-atom effects in brominated rhodamine dyes give increased triplet yields and quantum yields for the generation of singlet oxygen [φ(1O2)] [15-17] relative to unmodified rhodamines. [18] However, wavelengths of absorption are little changed relative to their light-atom counterparts. The brominated analogues still target the mitochondria and have increased phototoxicity toward cancer cells. [17] Accordingly, there is a need in the areas of PDT and PACT to identify new compounds useful in these methodologies.
One compound which has been used in photodynamic therapy studies is tetramethyl rosamine (TMR-O). While TMR-O has promise in that it has been shown to be transported into the cell cytoplasm, its ultimate ability to be effective is in doubt; as with other rosamine dyes, upon irradiation, TMR-O exhibits a high degree of fluorescence at the expense of singlet oxygen production, the species which damages tumor cells such that they die rather than replicate.
On yet another front in the treatment of cancer, multidrug resistance (MDR) of tumor cells, mediated by the plasma membrane protein P-glycoprotein (Pgp), is a major concern for treatment of primary, metastatic and recurrent cancer. [26-28] Pgp pumps a variety chemicals and chemotherapeutic agents from tumor cells, resulting in treatment failures. [26, 29-31] Tumor cell resistance to a wide assortment of chemotherapeutic agents can arise from exposure to a single drug making subsequent treatments ineffective. [26-27]
The mechanism by which Pgp overexpression is induced during exposure to chemotherapeutics or chemical agents is not fully understood, and may occur at the transcriptional level by mechanisms such as gene amplification, gene rearrangement, DNA methylation, promoter mutation or chromatin modification. [32, 33] With any one of these factors, transcription is the key for induction of Pgp and in some cases this could be a rapid response to intra/extracellular stimuli. [32] Development of therapeutic interventions at the transcriptional level could be advantageous. Currently, the most direct approach to inhibiting Pgp function in cancer is at the level of binding and/or the inhibition of ATP hydrolysis that Pgp is dependent upon for drug efflux from cells.
Many MDR reversal agents, including verapamil, cyclosporin A, and PSC833, have been examined to counteract the mechanisms of drug resistance. [34-36] However, these compounds have significant drawbacks, such as alterations in cell metabolism and their toxicity toward normal tissues. The therapeutic window for these compounds is severely restricted because the dose necessary for effective inhibition of Pgp, in many cases, exceeds the minimal toxic concentration in normal tissue. [26, 37,38] Ideally, Pgp modulators would be administered in combination with chemotherapeutic agent(s) to increase anti-cancer drug uptake, retention and effectiveness. However, concomitant administration of high doses of modulators and therapeutic doses of anti-cancer agents have resulted in unacceptable toxicity requiring chemotherapeutic dose reduction and ineffective treatment. [38]
One source of Pgp inhibitors might be derived from the cationic rhodamine dyes, such as rhodamine 123 (Rh123) and tetramethylrosamine (TMR-O), both structure given directly below. [39-41] These dyes are transport substrates for Pgp and have been used as fluorescent markers to determine the efficacy of Pgp modulators. [41,42] However, Rh123 and TMR-O do not inhibit Pgp function. [41,43]

While TMR-O is a Pgp substrate, it has not been completely clear why this is so. Furthermore, variations of TMR-O, such as its sulfur and selenium analogs, are not readily available, as sulfur and selenium analogs of xanthylium compounds (chalcogenoxanthylium compounds) are more difficult to prepare than xanthylium compounds. In particular, the selenium analogs are difficult to prepare with available methods.